Fe-Li-A1 anode composite and thermal battery containing the same

ABSTRACT

A solid anode composite material for use in thermal batteries that comprises about 65-85% by weight (about 34 to 40 atom percent) particulate iron, about 15-35% by weight (50 to 70 atom percent) lithium, and about 0.1-10% by weight (1.7 to 2.3 atom percent) aluminum. Lithium and Aluminum are only slightly or not alloyed with the particulate iron. The iron and/or the aluminum may be in the form of a powder. The aluminum may be in the form of lithium-aluminum alloy.

BACKGROUND OF THE INVENTION

Thermal batteries are thermal activated, primary reserve, hermeticallysealed power sources, generally consisting of series or series-parallelarrays of cells. Each cell is comprised of an anode, anelectrolyte-separator that is solid and no-conductive at roomtemperatures, a cathode and pyrotechnical means. The cell is activatedby providing sufficient heat to melt the electrolyte.

A variety of electrochemical systems are known for use in thermal cells.The electrolytes are generally mixtures of alkali metal halides, mostcommonly eutectic mixtures of LiCl—KCl (melting at about 352° C.) andLiCl—LiF—LiBr (melting at about 440° C.) although other fusible saltmixtures have been used, such as alkali metal thiocyanates. etc. Commoncathode materials, among others, are iron pyrite, cobalt sulfide,calcium chromate, copper chloride and copper oxide.

Typically, pure lithium metal is used as an anode, however, due to itshigh reactivity, some prominent disadvantages exist, among them is theformation of lithium nitride. This compound serves as a catalyst for itscontinuous formation, in particular, during nitrogen leakage into thebattery during aging period, resulted in a gradual conversion of themetallic lithium anode into said nitride. This phenomenon has been foundto seriously degrade the life time of a thermal battery. This problem oflithium nitride formation was never totally solved and elimination ofthis compound, remained a source of concern throughout the years.

Due to lithium's high reactivity the anode preparation requires verydifficult maintenance of high purity argon gas to prevent lithiumnitride formation. Even with expensive appropriate equipment, thelithium-based material, once it is formed, becomes tarnished aftercooling. The mat gray film formed as a result of the precipitation oflithium nitride and other impurities (R. Szwarc and S Dallek, “The Li(B)Ingot Preparation Scale-Up Study—Final Report”, GEPP-TM-645, GeneralElectric Company, 1982).

There are two reasons for lithium nitride formation during lithiummelting. One is that a “nitrogen free” atmosphere generally comprisesabout 1 ppm of nitrogen. This concentration, although very small, cannotbe considered as a “free” or “zero” nitrogen atmosphere. Said smallamount of nitrogen is enough for reacting with lithium to produce animpurities amount of lithium nitride in the lithium-based anodematerial. Further, nitride impurities are always present in the lithiumraw material. Such impurities, even in a very small quantity, areinevitable due to the existence of an eutectic composition betweenlithium and lithium nitride.

The eutectic composition comprises 0.068% mol nitrogen {P. Hubberstey,R. J. Pulham and A. E. Thunder, “Depression of the freezing point oflithium by nitrogen and by hydrogen”, J. Chem. Soc. Faraday Trans., 1[72] 431-435 (1976)} and always causes lithium raw material to containlithium nitride impurity.

U.S. Pat. No. 3,930,888 discloses active anode metals, including alkalimetals, alkaline earth metals or alloys thereof that melt below the celloperating temperature, or, for most purposes, below about 400° C.,preferably lithium or an alloy of lithium and calcium. Use of liquidlithium anode in thermal batteries provides a number of advantages amongthem are its capability of providing high voltage, power density andenergy density.

The active anode metal is carried by a foraminous metal substrate thatis wet by the molten anode metal and is substantially inert toelectrochemical or other reaction in the particular cell system used.The substrate is filled with active anode metal, most suitably bydipping the substrate in molten anode metal, withdrawing the substrateand then cooling it below the melting point of the anode metal; when theanode metal is melted on activation of the cell it will then wet andfill the substrate.

The anode housing comprises an impervious inert metal portion and aporous refractory fibrous portion. The metal portion is in electricalcontact with the anode metal and may be of any solid metal substantiallyinert to the other cell components with which it may contact, preferablynickel, stainless steel or iron. The porous portion (dry asbestos fibersand/or any insoluble, inorganic, non-metallic fibers of high meltingpoint that is infusible during operation of the cell, such as refractoryor ceramic fibers, either acidic, basic or amphoteric, may be used) ofthe housing is in tight engagement with the entire periphery of themetal portion of the housing in order to prevent leakage of the moltenanode metal along the metal housing surface to the exterior of thehousing, that would cause shorting or other premature failure.

A major disadvantage of this anode lies in the reactive nature oflithium and its low melting point (about 180° C.) which may result in aleakage of the molten metal, and consequently may cause short circuitsand premature failure in such batteries.

U.S. Pat. No. 4,221,849 relates to an anode material comprising apyrometallurgically combined iron-lithium anode for use in lithium anodethermal batteries. The ratio of lithium to iron is about 15% to 35%. Inthese ratios, the iron particles are held together by the surfacetension of the lithium rather than being alloyed thereto. The lithium isheated to about 500-600° F. and the iron added in particulate form whilestirring the molten mixture. The iron-lithium anode disk is positionedin a metal cup by means of an inert insulator or separator ringpreferably, made of Fiberbrax®. The electrolyte, normally in the form ofa wafer, is positioned adjacent to the separator in the cup.

It has been found that an activation of thermal batteries assembled withsaid iron-lithium anodes formed a noise of a few seconds duration, withpeak-to-peak values of greater than 0.5 volts (between 3 and 15 KHz).The noise which is greatly exaggerated in batteries operating in coldconditions as compared to those operating in warm or hot conditions, hasfound to seriously degrade the final activation rate of these batteries.

U.S. Pat. No. 4,675,257 which uses the same anode of the U.S. Pat. No.4,221,849 comprises a metal cup and a metal screen interposed betweenthe metal cup and anode composite material. The positioning of a metalscreen between the metal cup and the anode composite material togetherwith removal of the fiberfrax separator resulted in the reduction andelimination of activation noise and improved the electricalcharacteristics of the battery.

The above three US patents have the major disadvantage of undesireddevelopment of lithium nitride as discussed hereinabove.

U.S. Pat. No. 4,781,756 teaches a process for the removal of lithiumnitride from high purity lithium metal by adding a stochiometric amountof aluminum to liquid lithium metal containing lithium nitride (at atemperature between the melting point of lithium and 300° C.) to reactwith the lithium nitride, in an inert, nitrogen-free, atmosphere to formaluminum nitride, and subsequently separating the aluminum nitride fromthe liquid lithium metal by settling and filtering the mixture using a0.5 μm filter.

U.S. Pat. No. 5,019,158 deals with a process for separating calcium andnitrogen from lithium, in which alumina is added to a molten lithium andreacts to produce aluminum and lithium oxide. The aluminum reacts withthe nitrogen in the lithium to produce insoluble aluminum nitride, whilethe lithium oxide reacts with the calcium present to produce insolublecalcium oxide and lithium. The insoluble calcium oxide and aluminumnitride may then be separated from the molten lithium (for example, byfiltration). This operation is preferably takes place at temperaturesbetween 200° C. and 250° C.

The latest two discussed patents (U.S. Pat. Nos. 4,781,756 and5,019,158), suffer from the disadvantage of having a filtration stepwhich is very difficult to perform whenever high viscous mixtures exist.More specifically, they include a filtration step to remove theinsoluble oxides and nitrides from the molten lithium metal. As aresult, they cannot been used in the process for production ofiron-lithium anodes, due to the fact that the pyrometallurgicallycombinations of iron and lithium (15-35wt % Li) form highly viscousmixtures that are not filterable.

U.S. Pat. No. 4,158,720 discloses a lithium-aluminum-iron alloy for usein a negative electrode within a secondary electrochemical cell. It wasfound that such electrodes exhibit increased electrode potential overthat of electrodes containing only lithium-aluminum alloys. The anodecomposition comprises about 5-50 atom percent lithium and about 50-95atom percent alloy of aluminum and iron. The aluminum and iron alloyincludes about 20-35 atom percent iron.

The aluminum-iron alloy (Fe₂Al₅), when saturated with lithium, providesan increased lithium activity and consequently increased electrodevoltage over that of a comparable lithium-aluminum alloy. The electrodematerial is prepared by first providing an alloy of aluminum and ironand then electrochemically depositing lithium into a porous masscontaining that alloy.

There is a major drawback to this US patent that should be discussed.The anode material consists of an alloyed lithium compound (Li—Al—Fealloy), and consequently suffers from lithium not being present in anactive metal form. The absence of lithium in an active metal formresulted in a substantial reduction in the anode's potential.

SUMMARY OF THE INVENTION

It is the object of present invention to provide a new thermal batterycontaining a new Fe—Li—Al anode composite which includes the advantagesof lithium in an active metal form without sacrificing for lithiumpropensity of being nitrided in the presence of nitrogen.

It is a further object of present invention to provide a thermal batterycontaining a new Fe—Li—Al anode composite having longer shelf life withimproved characteristics. More specifically, the thermal batterycontaining the new anode of the invention shows no reduction in voltageor power as a result of formation of lithium nitride.

The anode composite of present invention comprises as much as 50 to 70atom percent of active lithium in metal form (compared to only 5 to 50atom percent of active lithium in the anode composite of the U.S. Pat.No. 4,158,720 discussed above). In addition, the anode of the presentinvention comprises about 34 to 40 atom percent of iron and only 1.7 to2.3 atom percent of aluminum (compared to 10 to 33 atom percent of ironand as much as 33 to 76 atom percent of aluminum in the anode compositeof the above mentioned US patent). Furthermore, the lithium and aluminumcomponents in the anode composite of present invention are not in analloy form but rather in a metal form, pyrometallurgically combined withiron powder, yielding the highest possible potential the lithium-basedanode could provide. However, a minor proportion of lithium and aluminummay be in an alloy form with iron powder.

Comparing U.S. Pat. No. 4,158,720 with the present invention reveals asignificant difference in the process of obtaining the Fe—Li—Al anodecomposite. Whereas the process in the US patent uses an Al—Fe alloy ofvarious compositions (such as, Fe₂Al₅, FeAl₃ and FeAl₂), as a componentof the anode, in the process of the present invention iron and aluminumare used as metal powders, thereby eliminating an additional step ofproduction of said alloys, which needs very high operationaltemperatures (about 1200° C.). Furthermore, whereas the process of theU.S. Pat. No. 4,158,720 applies an electrochemical deposition of lithiumin the formation of lithium alloy, the process of the present inventionapplies an addition of lithium in a particulate metal form, thusenabling keeping the lithium in the preferred metal form.

Yet a further object of present invention is to provide aniron-lithium-aluminum anode composite, in which whenever lithium nitrideis formed it is immediately reacted with aluminum, to be present in ametal form or in an alloy form (such as Li—Al alloy). It should bepointed out, however, that metals other than Al, as well (for example,gallium) may react with lithium nitride and may substitute for aluminumin the anode composite. Suitable metals are those the nitrides of whichare thermodynamically more stable than lithium nitride. The Li—Al meltmay pyrometallurgically combine with a particulate metal selected fromthe group containing iron, stainless steel, nickel and nichrome.Consequently, the iron in the Fe—Li—Al anode of present invention, maybe replaced by any of said metals.

In a preferred embodiment of present invention the anode configurationcomprises a metal cup containing the iron- lithium-aluminum anode inwhich a metal screen is interposed between the metal cup and the anodecomposite material, based on the assembly described in the U.S. Pat. No.4,675,257 discussed above.

The ratio of iron- lithium-aluminum anode composite of present inventionis about 65-85% by weight (about 34 to 40 atom percent) iron, about15-35% by weight (50 to 70 atom percent) lithium and about 0.1-10% byweight (1.7 to 2.3 atom percent) aluminum.

In a preferred mode, the lithium is heated to about 300-400° C. and thealuminum, in a metal or in lithium-aluminum alloy form, is added in aparticulate form while stirring the molten mixture for about 30 minutesor longer to allow the aluminum to react with lithium nitride. Followingaluminum addition, and without removing aluminum nitride or unreactedaluminum, iron powder is added while stirring the molten mixture forabout 30 minutes or longer, to obtain an homogenous mixture. The mixtureof lithium, aluminum and wetted iron powder is cooled in previouslyheated graphite molds. The entire process is conducted in a rare gasinert atmosphere, preferably argon.

It should be pointed out that despite the aluminum addition andconsequently, the formation of aluminum nitride, the anode compositematerial retains the essential characteristics of lithium and it iseasily rolled and shaped.

Unlike conventional thermal battery's lithium and lithium-iron anodes,the anode composite material of present invention, has reducedpropensity of being nitrided in contact with nitrogen atmospheres. Theanode of present invention thus maintains its active lithium in a metalform during the required prolonged storage periods of thermal batteries,while at the same time, blocking the lithium nitride formation.

Furthermore, it was unexpectedly found that the addition of aluminum ina lithium-aluminum alloy form to the anode composite reduces noiselevels during thermal battery activation. It has been speculated thatthe reduced levels of activation noise of battery of present inventionis due to lower surface tension value of lithium-aluminum melt comparingto the surface tension of lithium melt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general view of a common thermal battery comprising astacked array of electrochemical cells. It should be pointed out thatoptions of using mechanical instead of electrical activation, and usingpyrotechnic fuse strip instead of pyrotechnic fuse oil are not shown inthe figure;

FIG. 2 is an example of an anode assembly suitable for use in thepresent invention, as was demonstrated in U.S. Pat. No. 4,675,257. Sameassembly without the center opening may be used, as well;

FIGS. 3 and 4 show discharge behavior of cold and hot conditionedthermal batteries, respectively, using the anodes of the presentinvention. More specifically, FIGS. 3 and 4 demonstrate typical outputvoltage obtained from the discharge of thermal batteries assembled withanodes of the present invention. The conditioned temperatures are −54°C. and +71° C. (FIGS. 3 and 4, respectively). The upper and lower curvesrepresent the voltage output of 11 cells and 6 cells battery section,discharged at an average current density of 530 mA/cm² and 770 mA/cm²,respectively; and

FIGS. 5 and 6 show activation noise during first second discharge ofcold conditioned thermal batteries assembled with the anodes of theprior art, U.S. Pat. No. 4,675,257 (FIG. 5), and anodes of the presentinvention (FIG. 6). The three upper curves represent the output voltageobtained during batteries activation. A noise was observed in FIG. 5while in FIG. 6 the activation is noiseless and smooth.

EXAMPLES

About 140-200 gr of lithium in ingot form (99.9%, battery grade, FMCCorp. Lithium Division, NC, USA) or strip form (99.9%, battery grade,Tadiran, Battery Division, Israel) were introduced in a previouslyheated iron pot. After melted at 300° C.-400° C., 0.2-20 gr of −200+325mesh lithium-aluminum powder (20 wt % Li, Chemetall Foote Mineral Co.,NC, USA) or −40+325 mesh aluminum powder (99.8%, Alfa Aesar, USA) wasadded. Following 50 min of continuous stirring, 580-1050 gr of vacuumdried −200 mesh iron powder (TX-1000, Pfizer Overseas Inc., NY, USA;AS-1000, Rafael, Israel) was gradually added to the melt duringcontinuous stirring. After stirring for about 30 min to 1 hour, anhomogeneous mixture was obtained.

The mixture was cast into previously heated graphite molds and permittedto cool. After the composite material has cooled it was removed from themold as an ingot. After protective mineral oil (Silicaid AP-200, AidchimLtd., Israel) was applied to the ingots surface, they were transferredto a dry-room (<1% humidity) where they rolled and shaped into anodes.With exception of rolling and shaping steps, the entire process wascarried out in an argon purified system (Vacuum/Atmospheres Company, CA,USA) maintaining the total partial pressure of nitrogen and oxygenbellow 10 ppm (generally 1 ppm).

In the following example 1, lithium nitride was added during the processfor production of a known pyrometallurgically combined Li—Fe anode asdescribed in the U.S. Pat. No. 4,675,257 (defined hereinafter as “priorart”) and during the process for production of the new anode compositeof present invention. The number of nitrided anodes that were foundafter defined aging period, was determined and compared.

It was found that the number of nitrided anodes was significantly lowerwith the new anode of present invention.

Example 1

Prior art: 145 gr Li (ingot form)+1.8 gr Li₃N powder+683 gr Fe (NX-1000)were treated, as described in the U.S. Pat. No. 4,675,257. A smallquantity of lithium nitride (−80 mesh powder, Aldrich, USA) was added tosimulate infected raw lithium. The resultant ingots were rolled andshaped into 38 anodes of 0.25 gr weight and 25 mm diameter.

The process of present invention: 151 gr Li (ingot form)+1.8 gr Li₃Npowder+20 gr LiAl+711 gr Fe (NX-1000) were treated as described abovefor the process of present invention. A small quantity of lithiumnitride was added to simulate infected raw lithium. The resultant ingotswere rolled and shaped into 45 anodes of 0.25 gr weight and 25 mmdiameter.

Results: All anodes were aged during 32 days at 70° C. in an oven opento the dry-room atmosphere. After the aging period, 42% of anodes madeaccording to the prior art were nitrided while none of anodes madeaccording to the present invention were nitrided.

Example 2

Prior art: 149 gr Li (nitrided ingots)+679 gr Fe (NX-1000) were treatedas described in the U.S. Pat. No. 4,675,257. The resultant ingots wererolled and shaped into 260 anodes of 0.36 gr weight and 30 mm diameter.

The process of present invention—A: 149 gr Li (nitrided ingots)+25 grLiAl+680 gr Fe (NX-1000) were treated as previously described. Theresultant ingots were rolled and shaped into 286 anodes of 0.36 grweight and 30 mm diameter.

The process of present invention—B: 207 gr Li (strip form)+35 grLiAl+943 gr Fe (NX-1000) were treated as previously described. Theresultant ingots were rolled and shaped into 207 anodes of 0.36 grweight and 30 mm diameter.

Results: All anodes were aged at 70° C. in a furnace open to thedry-room atmosphere. Table 1 summarizes the aging results.

TABLE 1 Aging results of example 2. Number of nitrided anodes foundduring aging Aging Present invention Present invention period, daysPrior art A B 1 0 0 0 24 3 0 0 31 10 1 0 50 (7%) 18 (0.7%) 2 (0%) 0

Example 3

Prior art: 100 gr Li (strips)+420 gr Fe (AS-1000) were treated asdescribed in the U.S. Pat. No. 4,675,257. The resultant ingots wererolled and shaped into 100 anodes of 0.36 gr weight and 30 mm diameter.

The process of present invention—A: 150 gr Li (strips)+25 gr LiAl+630 grFe (AS-1000) were treated as previously described. The resultant ingotswere rolled and shaped into 100 anodes of 0.36 gr weight and 30 mmdiameter.

The process of present invention—B: 150 gr Li (strips)+20 gr Alpowder+630 gr Fe (AS-1000) were treated as previously described. Theresultant ingots were rolled and shaped into 100 anodes of 0.36 grweight and 30 mm diameter.

Results: All anodes were aged at 70° C. in a furnace open to thedry-room atmosphere. Table 2 summarizes the aging results.

TABLE 2 Aging results of example 3. Number of nitrided anodes foundduring aging Aging Present invention Present invention period, daysPrior art A B 1 0 0 0 21 — 0 0 40 (100%) 100 (0%) 0 (4%) 4

Case A (Li—Al) shows better resistance to the nitrogen attack comparedto Case B (Al powder). This difference in resistance between both casesis probably originated from a better dissolution properties of Li—Al inlithium, compared to aluminum powder.

Example 4

Prior art: 200 gr Li (strips)+1050 gr Fe (AS-1000) were treated aspreviously described. The resultant ingots were rolled into 0.3 mmthickness, 60 mm width composite strips.

The process of present invention—A: 200 gr Li (strips)+11 gr LiAl+1050gr Fe (AS-1000) were treated as previously described. The resultantingots were rolled into 0.3 mm thickness, 60 mm width composite strips.

The process of present invention—B: 200 gr Li (strips)+22 gr LiAl+1050gr Fe (AS-1000) were treated as previously described. The resultantingots were rolled into 0.3 mm thickness, 60 mm width composite strips.

The process of present invention—C: 200 gr Li (strips)+33 gr LiAl+1050gr Fe (AS-1000) were treated as previously described. The resultantingots were rolled into 0.3 mm thickness, 60 mm width composite strips.

Results: All strips were aged at 70° C. in a furnace open to thedry-room atmosphere. Strips made according to the prior art werenitrided after 1 day aging. Strips made according to the presentinvention—A were nitrided after 2 days aging. In contrast, strips madeaccording to the present invention—B and present invention—C were notnitrided even after more than 60 days aging.

In this example, cases A, B and C represent a use of different amountsof Li—Al (11, 22 and 33 gr, respectively). It was found that in case A,the amount of Li—Al added was too small for protecting the anode ofnitrogen attack. However, the amounts of Li—Al added in cases B and C,were higher and, consequently, have provided a much better protection ofthe anode against nitrogen attack. In cases B and C, the anodeswithstood nitrogen attack for more than 60 days aging.

1. A solid anode composite material containing lithium, for use inbatteries having longer shelf life, said composite material solidifiedfrom a melt obtained at a temperature of about 300-400° C., comprising:(a) about 65-85% by weight a metal chosen from iron and nickel; (b)about 15-35% by weight lithium; and (c) about 0.1-10% by weightaluminum; said aluminum essentially removing lithium nitride from saidcomposite material, and blocking the formation of lithium nitride insaid composite material.
 2. A solid anode composite according to claim1, wherein said aluminum comprised in a powder prior to said compositebeing solidified.
 3. A solid anode composite according to claim 1,wherein said metal is iron, said iron being in the form of powder priorto said composite being solidified.
 4. A thermal battery comprising asolid composite anode as defined in any one of claims 1 to
 3. 5. Athermal battery according to claim 4, exhibiting a reduced noise levelduring thermal activation.
 6. A thermal battery according to claim 4,exhibiting a prolonged storage life without the reduction in voltage andwithout its anode being nitrided.
 7. A thermal battery according toclaim 4, exhibiting essentially the highest potential that alithium-based anode can provide.
 8. A method for production of a solidanode composite as defined in claim 1, comprising: (a) providing aninert atmosphere; (b) introducing lithium into molds and allowing thelithium to melt; (c) allowing a metal capable of forming a nitridecompound thermodynamically more stable than lithium nitride, said metalbeing aluminum in a powder form, to mix with said molten lithium and toreact with lithium nitride impurities at a temperature of about 300-400°C. without filtering the melt; (d) allowing a metal chosen from amongiron and nickel, in a powder form, to mix with said melt in (c) at atemperature of about 300-400° C.; (e) allowing the composite in (d) tocool, followed by separation of the anode composite in the form of aningot from said molds; (f) optionally applying a protective coat to thesurface of the ingot in a conventional way; and (g) rolling and shapingthe anode into the desired shape.
 9. A method according to claim 8,wherein steps (a) to (f) are carried out in an argon purified system,wherein the total partial pressure of nitrogen and oxygen is maintainedbellow 50 ppm, preferably 10 ppm, and more preferably 1 ppm.
 10. Amethod according to claims 8, wherein said aluminum is provided in theform of a lithium-aluminum powder.